Pre-emphasis and De-emphasis in FM Broadcasting: Improving Audio Quality

Pre-emphasis and de-emphasis are essential signal processing techniques used in FM broadcasting to improve the overall signal-to-noise ratio (SNR) of the audio signal. Understanding these concepts is crucial for anyone setting up an FM broadcast system, as incorrect settings can result in unnatural-sounding audio.

The Problem: FM Noise Characteristics

In FM broadcasting, the noise power increases with frequency—a phenomenon known as the "triangular noise spectrum." This means that higher audio frequencies (treble) are more susceptible to noise than lower frequencies (bass). Without compensation, the listener would hear excessive hiss at high frequencies, especially in weak signal areas.

The Solution: Pre-emphasis and De-emphasis

Pre-emphasis is applied at the transmitter: higher audio frequencies are boosted (emphasized) before modulation, increasing their amplitude relative to lower frequencies. De-emphasis is applied at the receiver: the inverse filter attenuates the boosted high frequencies back to their original level, simultaneously reducing the high-frequency noise that was introduced during transmission. The result is a flat audio response with reduced overall noise.

Pre-emphasis Time Constants

The amount of pre-emphasis is defined by a time constant that determines the cutoff frequency of the boost filter:

• 50µs (microseconds): Used in Europe, Asia, Africa, and most of the world. Cutoff frequency: ~3.18kHz.
• 75µs: Used in North America (USA, Canada) and Japan. Cutoff frequency: ~2.12kHz. Provides more boost at high frequencies.
• 0µs (off): No pre-emphasis. Used in special applications or when external processing handles emphasis.

Setting Pre-emphasis on Elecsky Transmitters

All Elecsky FMT5.0 Pro series transmitters support selectable pre-emphasis via the LCD panel. Simply navigate to the pre-emphasis setting and choose 0µs, 50µs, or 75µs. Always set the pre-emphasis to match your region's broadcast standard:

• Europe, Asia, Africa, Australia: Select 50µs
• USA, Canada, Japan, South Korea: Select 75µs
• If using external audio processing: Select 0µs and let the processor handle emphasis

Using the wrong pre-emphasis setting will cause audio to sound too bright (excessive treble) or too dull (muffled), and may also cause overmodulation at high frequencies. Always verify the setting before beginning broadcasts.

Technical Implementation

Pre-emphasis is implemented as a simple first-order RC high-pass filter with a time constant determined by the RC product. At the transmitter, the filter boosts frequencies above the cutoff point at a rate of 6dB per octave. At the receiver, an identical but inverse filter (de-emphasis) restores the original frequency balance while attenuating high-frequency noise by the same amount. This complementary process ensures transparent audio reproduction with significantly improved signal-to-noise ratio at higher frequencies.